A liquid crystal display (LCD) typically includes a unit having two glass substrates (or other types of substrates) that face each other, with a liquid crystal layer sandwiched between the substrates. An LCD in a vertically aligned (VA) mode uses negative liquid crystal material and vertically aligned film. When not supplied with a voltage, the liquid crystal molecules in the liquid crystal layer are arranged in a vertical direction (with respect to the main surfaces of the substrates) and the VA LCD cannot be penetrated by an incident light, resulting in a dark display. When supplied with a preset voltage, liquid crystal molecules are arranged in a horizontal direction and the VA LCD can be penetrated by an incident light, resulting in a white display.
However, when viewed at an angle not perpendicular to the display, a user may perceive a contrast reduction or contrast reversal problem with a VA LCD. This is the result of interaction of the light with the liquid crystal molecules within the LCD. When traveling through the LCD in a direction that is not at a right angle of incidence, the light interacts with the liquid crystal molecules in a way different from that when the light travels through the LCD in a direction at a right angle of incidence. Therefore, the contrast between the state when the light penetrates (white) and the state when the light does not (black) will drop significantly when the light is not at a right angle of incidence. This results in unsatisfactory performance of VA LCDs in many applications (e.g., flat television displays and large computer displays).
A larger viewing angle can be provided by an LCD in MVA (multi-domain vertical alignment) mode. In an MVA LCD, improvement in viewing angle is achieved by setting the orientations of the liquid crystal molecules within each pixel of the display to a plurality of different directions. In some conventional MVA LCDs, a multi-domain regulation is provided to improve the display's performance at various viewing angles. Typically, this multi-domain regulation is achieved by providing a plurality of slits in the pixel electrode of the thin film transistor substrate and a plurality of protrusions at the common electrode of the color filter substrate, where the protrusions and slits are arranged in an alternating fashion. The aligned orientation of the liquid crystal molecules depends on the fringe field produced by the pattern of the protrusions and slits.
To drive an LCD, a voltage is applied to cause the corresponding liquid crystal molecules within each pixel to switch. The switching of the molecules will change the light transmittance of each pixel. In response to switching of the liquid crystal molecules, the LCD will provide different brightness. For most LCDs, the higher the applied voltage, the quicker the response if the initial voltage is kept constant. However, this quicker response time at higher applied voltages may not be true with certain LCDs, such as LCDs in the patterned vertical alignment (PVA) mode and MVA mode. In such LCDs, under certain circumstances, the LCDs may respond slower when a higher voltage is applied.
FIG. 11A shows an arrangement of protrusions and slits in the pixel area of a conventional MVA LCD. FIG. 11B shows a cross-section along line A-A in FIG. 11A. As depicted in FIG. 11A, a pixel area 400 of the LCD is defined generally near the intersection of a gate line 402 and a data line 404. The pixel area 400 has a thin film transistor (TFT) 406, which is electrically connected to the gate line 402 and the data line 404. The pixel area 400 also contains a pixel electrode 419 that is connected to the TFT 406.
As depicted in FIG. 11B (cross section along line A-A′ in FIG. 11A), protrusions 410 and slits 412, provided in the pixel area 400, are formed in a color filter substrate 414 and a TFT substrate 416, respectively. The arrangement of protrusions 410 and slits 412 depicted in FIGS. 11A-11B causes the liquid crystal molecules and the penetration axis of the upper and lower polarizers (not shown) to be oriented such that the liquid crystal molecules and the penetration axis of the upper and lower polarizers form an angle of 45°, which enables the MVA LCD to provide maximum gray scale brightness due to light traveling through the MVA LCD. However, when the orientation of the liquid crystal molecules and the penetration axis of the upper and lower polarizers (not shown) fail to form an angle of 45° under regulation of protrusion 410 and slit 412, which may occur when a gap between protrusions and slits becomes too large, disclination of the liquid crystal molecules occurs. In a liquid crystal cell, disclination refers to the orientation of the liquid crystal molecules changing uncontinuously at a point or a line. Disclination of liquid crystal molecules results in the MVA LCD not being able to provide maximum gray scale brightness.
FIGS. 12A-E simulate the switching of the liquid crystal molecules in area 419 of FIG. 11A. The switching, which is observed within the same time duration, is caused by the fringe field produced by the pattern of the protrusions and slits when different voltages are applied. The transverse axis and vertical axis in FIGS. 12A-E correspond to directions A-A′ and A-B of FIG. 11A, respectively. As shown in FIGS. 12A and 12B, when the voltage applied is 5V and 5.5V. respectively, the liquid crystal molecules in area 418 are arranged by the fringe field in a normal pattern. However, as shown in FIGS. 12C-E, when the voltage applied rises to 5.75V, 6.0V, and 6.5V, liquid crystal molecules in several regions (e.g., 420a and 420b) of the area 418 will not be arranged by the fringe field, and as a result, disclination occurs in regions 420a and 420b. Disclination is worse in regions 420a and 420b of FIG. 12E, which shows the result of an applied voltage of 6.5V.